Feedback is a well known problem in hearing devices and systems for suppression and cancellation of feedback are well-known in the art, see e.g., U.S. Pat. No. 5,619,580, U.S. Pat. No. 5,680,467 and U.S. Pat. No. 6,498,858.
Conventionally, a Digital Feedback Suppression Circuit is employed in hearing devices to suppress the feedback signal from the receiver output. During use, the Digital Feedback Suppression Circuit estimates the feedback signal, e.g. utilising one or more digital adaptive filters that model the feedback path. The feedback estimate from the Digital Feedback Suppression Circuit is subtracted from the microphone output signal to suppress the feedback signal.
The feedback signal may propagate from the receiver back to the microphone along an external signal path outside the hearing device housing and along an internal signal path inside the hearing device housing.
External feedback, i.e. propagation of sound from the receiver to the microphone of the hearing device along a path outside the hearing device, is also known as acoustical feedback. Acoustical feedback occurs, e.g., when a hearing device ear mould does not completely fit the wearer's ear, or in the case of an ear mould comprising a canal or opening for, e.g., ventilation purposes. In both examples, sound may “leak” from the receiver to the microphone and thereby cause feedback.
Internal feedback may be caused by sound propagating through air inside the hearing device housing, and by mechanical vibrations in the hearing device housing and in components inside the hearing device housing. The mechanical vibrations are generated by the receiver and are transmitted to other parts of the hearing device, e.g. through receiver mounting(s). In some hearing devices, the receiver is flexibly mounted in the housing, whereby transmission of vibrations from the receiver to other parts of the hearing device is reduced.
WO 2005/081584 discloses a hearing device having two separate digital feedback suppression circuits, namely one for compensation of the internal mechanical and acoustical feedback and one for compensation of the external feedback.
The external feedback path extends “around” the hearing device and is therefore usually longer than the internal feedback path, i.e. sound has to propagate a longer distance along the external feedback path than along the internal feedback path to get from the receiver to the microphone. Accordingly, when sound is emitted from the receiver, the part of it propagating along the external feedback path will arrive at the microphone with a delay in comparison to the part propagating along the internal feedback path. Therefore, it is preferred that the separate digital feedback suppression circuits operate on first and second time windows, respectively, and that at least a part of the first time window precedes the second time window. Whether the first and second time windows overlap or not, depends on the length of the impulse response of the internal feedback path.
While external feedback may vary considerably during use, internal feedback is more constant and typically coped with during the manufacturing process.
It is well-known that accurate initialisation of the Digital Feedback Suppression Circuit is essential for effective suppression of feedback in the hearing device. Although in principle, an adaptive filter automatically adapts to changes of the feedback path, there are limitations to the extent and accuracy of feedback path changes that the adaptive filter can track. However, accurate initialization of the Digital Feedback Suppression Circuit leads to fast and accurate modelling of the feedback path response and effective feedback suppression during subsequent operation by provision of a starting point for the adaptation that is close to the desired end result. The initialisation may take place during a fitting session and possibly whenever the user turns the hearing device on.
Typically, the Digital Feedback Suppression Circuit is initialized during fitting of the hearing device to a specific user. The hearing device is connected to a PC, and a probe signal is transmitted to the receiver, and based on the microphone output signal that includes a response to the probe signal, the impulse response of the feedback path is estimated. Typically, the probe signal is 10 seconds long and has a high level that disturbs the user. In order to allow the user to adapt to the probe signal, the probe signal is ramped linearly on a logarithmic scale from zero during one second preceding the ten seconds constant signal level of the probe signal. The received microphone output signal is transmitted to the PC and the respective impulse response is calculated. Then the PC determines the parameters required by the Digital Feedback Suppression Circuit, e.g. filter coefficients of fixed digital filters and initial filter coefficients of an adaptive digital filter, to be capable of modelling the feedback path.
In a hearing device with more than one microphone, e.g. having a directional microphone system, the hearing device may comprise separate Digital Feedback Suppression Circuits for each microphone that are initialised separately utilising the same probe signal.
Hearing device users have complained about discomfort and pain during the initialisation process.
Recently, open solutions have emerged. In accordance with hearing device terminology, a hearing device with a housing that does not obstruct the ear canal when the housing is positioned in its intended operational position in the ear canal; is categorized “an open solution”. The term “open solution” is used because of the passageway between a part of the ear canal wall and a part of the housing allowing sound waves to escape from behind the housing between the ear drum and the housing through the passageway to the surroundings of the user. With an open solution, the occlusion effect is diminished and preferably substantially eliminated.
Typically, a standard sized hearing device housing which fits a large number of users with a high level of comfort represents an open solution.
Open solutions may lead to feedback paths with long impulse responses, since the receiver output is not separated from the microphone input by a tight seal in the ear canal. This makes the feedback path relatively open leading to a long impulse response which may further increase the required duration of the probe signal for estimation of the feedback path.
Thus, it is desirable to provide a way of initialising the Digital Feedback Suppression Circuit that reduces user discomfort during the initialisation process.
EP 2 205 005 A1 discloses a hearing instrument with digital feedback suppression circuitry having parameters that are initialised, e.g. during fitting of the hearing instrument to a specific user, according to a method of modelling a feedback path from a receiver to a microphone of the hearing instrument, comprising the initialisation steps of transmitting an electronic probe signal to the receiver for conversion into an acoustic probe signal output by the receiver while recording the microphone output signal, and determining at least one parameter of the feedback path based on the recorded microphone output signal, and wherein the step of transmitting a probe signal to the receiver comprises the steps of increasing the level of the probe signal while monitoring values of a first quality parameter calculated based on the recorded microphone output signal, and refraining from further increasing the level of the probe signal when the determined first quality parameter has reached a predetermined first threshold value.